The heat-sensitive recording paper means sheets wherein images are formed by utilizing a physical or chemical change of substances by means of thermal energy.
Examples of heat-sensitive recording paper utilizing a physical change of substances by heat is the so-called wax type heat-sensitive recording sheet which is utilized for electrocardiograms. Heat-sensitive recording papers which utilize a chemical change by heat include those utilizing coloration or discoloration of a specified compound by heat and those utilizing a color reaction of two or more substances by heat. Examples of heat-sensitive recording papers utilizing a color reaction of two or more substances include those using a combination of ferric salt of higher fatty acids such as stearic acid and polyhydric hydroxy aromatic compounds (U.S. Pat. Nos. 2,663,654 to 2,663,657), those forming azo dyes or oxazine dyes from dye forming material (Japanese Patent Publication No. 9240/63) and those using a combination of a colorless dye such as Crystal Violet lactone, etc., and a phenol compound (Japanese Pat. No. 4160/68 and U.S. Pat. Nos. 3,451,338, 3,539,375, 3,674,535 and 3,666,525), etc.
Heat-sensitive recording papers do not require development because of primary coloration. Therefore, they are advantageous in that the recording apparatus is light and small-sized. Consequently they have been broadly utilized recently. On the other hand, they are undesirable because the recording speed is not sufficiently high. The response speed of the recording element is limited because the recording energy is heat. In order to overcome this drawback, various efforts have been made in the areas of both the recording apparatus and the recording paper. One method involves increasing the smoothness of a surface of the heat-sensitive recording paper as described in Japanese Patent Publication No. 20142/77 and Japanese Patent Application (OPI) No. 47351/73 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). Generally, in order to increase the smoothness of the heat-sensitive recording paper, a surface treatment is carried out by supercalendering, by which heat-transmission efficiency between the recording element and the heat-sensitive recording paper is improved. This contributes to an increase of recording speed.
However, surface treatment for improving smoothness has various drawbacks. One drawback is fogging. More specifically, a color reaction occurs during the surface treatment by which the recording paper is slightly colored. As a counterplan, it has been proposed to add granular waxes (Japanese Patent Publication No. 14531/75). However, waxes generally have a large heat capacity and high heat of fusion, by which heat-response of the heat-sensitive recording paper deteriorates, causing problems.
Another drawback of increasing smoothness is that the ability to write on the paper with pencils or ball point pens, etc., deteriorates. This drawback is expected because writing depends upon friction between the writing means and the paper. Still another drawback is difficulty during the processing step. More specifically, production efficiency is greatly deteriorated because supercalendering is carried out on off-machine in most cases but not carried out continuously into a coating step. Yet another drawback is that adhesion of the recording paper to the recording element occurs on the developed parts, because the recording element closely contacts with the recording paper, by which the running property deteriorates due to sticking.
In spite of various drawbacks, the fact is that the surface treatment to increase smoothness is nearly always carried out, because the improvement of smoothness comparatively contributes to the improvement of the recording speed.